1. Field of the Invention
The present invention relates to a method and apparatus for adjusting channel insertion timing, preferably applied to digital signal communication in a so-called integrated services digital network (ISDN).
2. Description of the Prior Art
As technology has advanced, the public has begun demanding a wider range of services from communication systems. One result of this has been the development of the ISDN. An important component of an ISDN is the so-called "network termination unit". A network termination unit functions as an interface between an external line and a plurality of terminal equipment units. The terminal equipment units are connected by a bus connection to the network termination unit via a common receiving (R) line and a common transmitting (T) line. Each terminal equipment unit receives digital signals provided from the network termination unit via the common R line. The digital signals are arranged into frame signals. Each frame signal is composed of successive channels. Each terminal equipment unit achieves communication with the network termination unit via one of the channels allotted thereto. Each terminal equipment unit also transmits digital signals to the network termination unit via the common T line. The transmitted digital signals are also arranged into frame signals. Therefore, each terminal equipment achieves communication with the network termination unit via the channel allotted thereto.
In each frame signal, the channels must be arranged sequentially without overlap. If one channel overlaps another, correct information transmission cannot be guaranteed for the two terminal equipment units concerned. To avoid such a channel overlap, in the prior art, large so-called, "guard bit areas" have been inserted between two adjacent channels.
There are problems, however, with the prior art method of guard bit areas. First, it is undesirable to insert a large number of guard bits into each frame signal since they do not themselves carry any information. Second, the longer the distance between the network termination unit and the terminal equipment unit, the larger the guard bit areas must be. Third, it is not easy for the network termination unit to reproduce a clock timing signal from a transmitted frame signal due to the presence of large guard bit areas.